Facet arthoplasty devices and methods

ABSTRACT

Surgically installed prostheses replace either the caudal portion of a natural facet joint, the cephalad portion of a natural facet joint, or both. The prostheses are readily attached to the pedicles of a vertebral body and support at least one element that defines an artificial facet joint structure. The caudal facet joint structure is sized and located to articulate with the cephalad facet joint structure. Together, the prostheses form a total facet replacement system. The system is suitable for use in virtually all levels of the spine.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/740,947, filed Jan. 14, 2013, which is acontinuation of U.S. patent application Ser. No. 12/806,410, filed onAug. 12, 2010, now issued as U.S. Pat. No. 8,377,103, which is acontinuation of U.S. patent application Ser. No. 11/957,149, filed Dec.14, 2007, which is a continuation of U.S. patent application Ser. No.10/961,726, filed Oct. 8, 2004, which is a divisional of U.S. patentapplication Ser. No. 10/067,137, filed Feb. 4, 2002, now U.S. Pat. No.6,811,567, which is a continuation-in-part of U.S. patent applicationSer. No. 09/693,272, filed Oct. 20, 2000, now U.S. Pat. No. 6,610,091,which claims the benefit of Provisional Patent Application Ser. No.60/160,891, filed Oct. 22, 1999, all of which are incorporated herein byreference.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

FIELD OF THE INVENTION

The present invention generally relates to devices and surgical methodsfor the treatment of various types of spinal pathologies, Morespecifically, the present invention is directed to several differenttypes of facet joint replacement prostheses, surgical procedures forperforming facet joint replacements, and surgical instruments which maybe used to perform the surgical procedures.

BACKGROUND OF THE INVENTION

Back pain is a common human ailment. In fact, approximately 50% ofpersons who are over 60 years old suffer from lower back pain. Althoughmany incidences of back pain are due to sprains or muscle strains whichtend to be self-limited, some back pain is the result of more chronicfibromuscular, osteoarthritic, or ankylosing spondolytic processes ofthe lumbosacral area. Particularly in the population of over 50 yearolds, and most commonly in women, degenerative spine diseases such asdegenerative spondylolisthesis (during which one vertebra slides forwardover the top of another vertebra) and spinal stenosis (during which thespinal canal markedly narrows) occurs in a high percentage of thepopulation. Iida, et al, 1989.

Degenerative changes of the adult spine have traditionally beendetermined to be the result of the interrelationship of the three jointcomplex: the disk and the two facet joints. Degenerative changes in thedisc lead to arthritic changes in the facet joint and vice versa. SeeFarfan and Sullivan, 1967; see also Farfan, 1969; see also Farfan, 1980.

One cadaver study of nineteen cadavers with degenerativespondylolisthesis showed that facet degeneration was more advanced thandisc degeneration in all but two cases. Farfan. In mildspondylolisthetic cases, the slip appeared to be primarily the result ofpredominantly unilateral facet subluxation. Other studies intodegenerative changes of the spine have revealed extensive contributionof facet joint degeneration to degenerative spinal pathologies such asdegenerative spondylolisthesis, central and lateral stenosis,degenerative scoliosis (i.e., curvature of the spine to one side), andkypho-scoliosis, at all levels of the lumbar spine. See Kirkaldy-Williset al, 1978; see also Rosenberg, 1975.

It has been determined that facet joint degeneration particularlycontributes to degenerative spinal pathologies in levels of the lumbarspine with sagittally oriented facet joints, i.e. the L4-L5 level.

When intractable pain or other neurologic involvement results from adultdegenerative spine diseases, such as the ones described above, surgicalprocedures may become necessary. Traditionally, the surgical managementof disease such as spinal stenosis consisted of decompressivelaminectomy alone. Herkowitz, et al, The Diagnosis and Management ofDegenerative Lumber Spondylolisthesis, 1998. Wide decompressivelaminectomies remove the entire lamina, and the marginal osteophytesaround the facet joint. Because a lot of degenerative spine disease hasbeen demonstrated to be caused by facet joint degeneration or disease,this procedure removes unnecessary bone from the lamina and insufficientbone from the facet joint.

Furthermore, although patients with one or two levels of spinal stenosistend to do reasonably well with just a one to two level widedecompressive laminectomy, patients whose spinal stenosis is associatedwith degenerative spondylolisthesis have not seen good results.Lombardi, 1985. Some studies reported a 65% increase in degree ofspondylolisthesis in patients treated with wide decompressivelaminectomy. See Johnson et al; see also White and Wiltse. The increasein spinal slippage especially increased in patients treated with threeor more levels of decompression, particularly in patients with radicallaminectomies where all of the facet joints were removed.

To reduce the occurrence of increased spondylolisthesis resulting fromdecompressive laminectomy, surgeons have been combining laminectomies,particularly in patients with three or more levels of decompression,with multi-level arthrodesis, which surgically fuses the facet joints toeliminate motion between adjacent vertebrae. Although patients whoundergo concomitant arthrodesis do demonstrate a significantly betteroutcome with less chance of further vertebral slippage afterlaminectomy, arthrodesis poses problems of its own. Aside from theoccurrence of further spondylolisthesis in some patients, additionaleffects include non-unions, slow rate of fusion even with autografts,and significant morbidity at the graft donor site. Furthermore, even ifthe fusion is successful, joint motion is totally eliminated at thefusion site, creating additional stress on healthy segments of the spinewhich can lead to disc degeneration, herniation, instabilityspondylolysis, and facet joint arthritis in the healthy segments.

An alternative to spinal fusion has been the use of an invertebral discprosthesis. There are at least 56 artificial disc designs which havebeen patented or identified as being investigated. McMillin C. R. andSteffee A. D., 20th Annual Meeting of the Society for Biomaterials(abstract)(1994). Although different designs achieve different levels ofsuccess with patients, disc replacement mainly helps patients withinjured or diseased discs; disc replacement does not address spinepathologies such as spondylolisthesis and spinal stenosis caused byfacet joint degeneration or disease.

SUMMARY OF THE INVENTION

There is a need in the field for prostheses and prosthetic systems toreplace injured and/or diseased facet joints, which cause, or are aresult of, various spinal diseases. There is also a need for surgicalmethods to install such prostheses. There is also a need for prosthesesand prosthetic systems to replace spinal fusion procedures.

The present invention overcomes the problems and disadvantagesassociated with current strategies and designs in various treatments foradult spine diseases. The present inventive spinal arthroplastic systemsavoid the problems of spine stiffness, increased loads on unfusedlevels, and predictable failure rates associated with spinalarthrodesis. The present invention pertains to spinal prosthesesdesigned to replace facet joints and/or part of the lamina at virtuallyall spinal levels including L1-L2, L2-L3, L3-L4, L4-L5, L5-S-1, T11-T12,and T12-L1. Various types of joint replacement prostheses are describedfor treating different types of spinal problems.

As will be described in greater detail later, a given natural facetjoint has a superior half and an inferior half. In anatomical terms, thesuperior half of the joint is formed by the vertebral level below thejoint (which can thus be called the caudal portion of the facet joint,i.e., because it is near the feet). The inferior half of the joint isformed by the vertebral level above the joint (which can thus be calledthe cephalad portion of the facet joint, i.e., because it is near thehead). For example, in the L4-L5 facet joint, the caudal portion of thejoint is formed by boney structure on the L-5 vertebra (e.g., a superiorarticular suface and supporting bone on the L-5 vertebra), and thecephalad portion of the joint is formed by honey structure on the L-4vertebra (e.g., an inferior articular surface and supporting bone on theL-4 vertebra).

For the sake of description, the prostheses that embody the features ofthe invention will be called either “cephalad” or “caudad” with relationto the portion of a given facet joint they replace. Thus, a prosthesisthat, in use, replaces the caudal portion of a facet joint (i.e., thesuperior half) will be called a “caudal” prosthesis. Likewise, aprosthesis that, in use, replaces the cephalad portion of a facet joint(i.e., the inferior half) will be called a “cephalad” prosthesis.

One aspect of the invention provides a facet joint prosthesis toreplace, on a vertebral body, a caudal portion of a natural facet joint(e.g., a superior articular surface and supporting bone structure on thevertebral body). The caudal prosthesis comprises a component sized to befixed to the vertebral body, e.g., on or near a pedicle. The caudalprosthesis includes an artificial facet joint structure adapted toreplace a caudal portion of the natural facet joint after its removalfrom the vertebral body. The removal of a caudal portion of the naturalfacet joint and its total replacement by the artificial facet jointstructure of the caudal prosthesis frees the orientation of theprosthesis from anatomic constraints imposed by a preexisting articularconfiguration of the caudal portion of the natural facet joint.Furthermore, the artificial facet joint structure of the caudalprosthesis can comprise an artificial articular configuration that isunlike the preexisting articular configuration, so that a desiredarticulation or bony anatomy can be restored to achieve a desired levelof function.

This aspect of the invention also provides a method of replacing, on avertebral body, a caudal portion of a natural facet joint. The methodremoves a caudal portion of the natural facet joint from the vertebralbody, and, in its place, fixes a component to the vertebral body thatincludes an artificial facet joint structure adapted to replace theremoved caudal portion of the natural facet joint. Desirably, theartificial facet joint structure includes an artificial articularconfiguration unlike the preexisting articular configuration of theremoved caudal portion of the natural facet joint.

Another aspect of the invention provides a facet joint prosthesis toreplace, on a vertebral body, a cephalad portion of a natural facetjoint (e.g., an inferior articular surface and supporting bone structureon the vertebral body). The cephalad prosthesis comprises a componentsized to be fixed to the vertebral body, e.g., on or near a pedicle, oron or near a lamina, or on or near a spinous process, or combinationsthereof. The cephalad prosthesis includes an artificial facet jointstructure adapted to replace a cephalad portion of the natural facetjoint after its removal from the vertebral body. As with the removal andtotal replacement of a caudal portion of the natural facet joint, theremoval of a cephalad portion of the natural facet joint and its totalreplacement by the artificial facet joint structure of the cephaladprosthesis makes possible the orientation of the prosthesis free fromanatomic constraints imposed by a preexisting articular configuration ofthe cephalad portion of the natural facet joint. Furthermore, like thecaudal prosthesis, the artificial facet joint structure of the cephaladprosthesis can comprise an artificial articular configuration that isunlike the preexisting articular configuration of the natural facetsurface (which is removed), so that a desired articulation or bonyanatomy can be totally restored.

This aspect of the invention also provides a method of replacing, on avertebral body, a cephalad portion of a natural facet joint. The methodremoves a cephalad portion of the natural facet joint from the vertebralbody, and, in its place, fixes a component to the vertebral body thatincludes an artificial facet joint structure adapted to replace theremoved cephalad portion of the natural facet joint. Desirably, theartificial facet joint structure includes an artificial articularconfiguration unlike the preexisting articular configuration of theremoved cephalad portion of the natural facet joint Another aspect ofthe invention provides a prosthesis assembly and related method forreplacing a natural facet joint between adjoining first vertebral bodyand a second vertebral bodies. The assembly and method utilize a firstcomponent sized to be fixed to the first vertebral body, which issuperior to the second vertebral body. The first component includes afirst artificial facet joint structure adapted to replace a cephaladportion of the natural facet joint on the first vertebral body afterremoval of the cephalad portion of the natural facet joint from thefirst vertebral body. The assembly and method also comprise a secondcomponent sized to be fixed to the second vertebral body. The secondcomponent includes a second artificial facet joint structure adapted toreplace the caudad portion of the natural facet joint of the secondvertebral body after removal of the caudad portion of the natural facetjoint from the second vertebral body. Together, the first and secondartificial facet joint structures comprise an artificial facet joint.The removal of both cephalad and caudal portions of a natural facetjoint and their total replacement by the artificial facet jointstructures of the first and second components allows the artificialfacet joint to be installed without anatomic constraints imposed by apreexisting articular configuration of the natural facet joint. Theremoval of the natural facet joints also removes bone spurs affectingthe nerve roots adjacent the natural facet structures, alleviating oreliminating pain. Furthermore, the artificial facet joint structures ofeither the first or second components, or both, can create an artificialarticular configuration for the artificial facet joint that is unlikethe preexisting articular configuration of the removed natural facetjoint, so that a desired articulation or bony anatomy can be completelyrestored. By restoring facet joint articulation to a desired level offunction, the incidence of spondylolisthesis can be reduced oreliminated. Other features and advantages of the inventions are setforth in the following Description and Drawings, as well as in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe claims that follow. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which.

In the drawings:

FIG. 1 is a lateral view of the L4 and L5 vertebrae;

FIG. 2 is a superior view of the L4 and L5 vertebrae in a separatedcondition;

FIG. 3 is a superior section view of a vertebral body, showing thefixation of a caudal prosthesis by pedicle screws to a vertebral body,the prosthesis carrying facet joint structure elements that createartificial facet surfaces for the superior half of a facet jointreplacement, the artificial facet joint structure elements in FIG. 3having a generally cup-shaped geometry and being shown in positionsmedial of the typical anatomic position of the superior half of thefacet joint;

FIG. 4 is a superior section view of a vertebral body, showing thefixation of a caudal prosthesis of the type shown in FIG. 3, the lengthof the prosthesis spanning the pedicles being adjustable, the cup-shapedartificial facet joint structure elements in FIG. 4 being shown inalternative positions either medial or lateral of the typical anatomicposition of the superior half of the facet joint;

FIG. 5 is a superior section view of a vertebral body, showing thefixation of a caudal prosthesis of the type shown in FIG. 3, theartificial facet joint structure elements in FIG. 5 having a generallyball-shaped geometry and being shown in positions medial of the typicalanatomic position of the superior half of the facet joint;

FIG. 6 is an enlarged perspective view of a caudal prosthesis of thetype shown in FIG. 3, showing a mechanism that allows movement of theartificial facet joint structure elements to adjust their relativespaced-apart positions on the prosthesis;

FIG. 7 is a superior section view of a vertebral body, showing thefixation of a caudal prosthesis by pedicle stems to a vertebral body,the prosthesis carrying facet joint structure elements that createartificial facet surfaces for the superior half of a facet jointreplacement, the artificial facet joint structure elements in FIG. 7having a generally cup-shaped geometry and being shown in positions ator near the typical anatomic position of the superior half of the facetjoint;

FIG. 8 is a superior section view of a vertebral body, showing thefixation of a caudal prosthesis of the type shown in FIG. 7, theartificial facet joint structure elements in FIG. 8 having a generallyball-shaped geometry; and

FIG. 9 is a perspective posterior view of the spinal column, showing thefixation of a cephalad prosthesis across the lamina of a vertebral body,the cephalad prosthesis carrying facet joint structure elements thatcreate artificial facet surfaces for the inferior half of a facet jointreplacement, FIG. 9 also showing the fixation of a caudal prosthesis ofthe type shown in FIG. 3 carrying facet joint structure elements thatcreate artificial facet surfaces for the superior half of a facet jointreplacement, the two prosthesis together created a total facetreplacement system.

The invention may be embodied in several forms without departing fromits spirit or essential characteristics. The scope of the invention isdefined in the appended claims, rather than in the specific descriptionpreceding them. All embodiments that fall within the meaning and rangeof equivalency of the claims are therefore intended to be embraced bythe claims.

DETAILED DESCRIPTION OF THE INVENTION

I. Anatomy of Lumbar Vertebrae

FIGS. 1 and 2 show the fourth and fifth lumbar vertebrae L4. and L5,respectively, in a lateral view (while in anatomic association) and in asuperior view (separately). The lumbar vertebrae (of which there are atotal of five) are in the lower back, also called the “small of theback.”

As is typical with vertebrae, the vertebrae L4 and L5 are separated byan intervertebral disk 25. The configuration of the vertebrae L4 and L5differ somewhat, but each (like vertebrae in general) includes avertebral body 10, which is the anterior, massive part of bone thatgives strength to the vertebral column and supports body weight. Thevertebral arch 12 is posterior to the vertebral body 10 and is formed bythe right and left pedicles 14 and lamina 16. The pedicles 14 are short,stout processes that join the vertebral arch 12 to the vertebral body10. The pedicles 14 project posteriorly to meet two broad flat plates ofbone, called the lamina 16.

Seven other processes arise from the vertebral arch. Three processes—thespinous process 18 and two transverse 20 processes—project from thevertebral arch 12 and afford attachments for back muscles, forminglevers that help the muscles move the vertebrae. The remaining fourprocesses, called articular processes, project superiorly from thevertebral arch (and are thus called the superior articular processes 22)and inferiorly from the vertebral arch (and are thus called the inferiorarticular processes 24).

The superior and inferior articular processes 22 and 24 are inopposition with corresponding opposite processes of vertebrae superiorand inferior adjacent to them, forming joints, called zygapophysialjoints or, in short hand, the facet joints or facets. The facet jointspermit gliding movement between the vertebrae L4 and L5. Facet jointsare found between adjacent superior and inferior articular processesalong the spinal column.

The facet joints can deteriorate or otherwise become injured ordiseased, causing lack of support for the spinal column, pain, and/ordifficulty in movement.

As described in this Specification, a facet joint has a superior halfand an inferior half. The superior half of the joint is formed by thevertebral level below the joint (which will accordingly be called thecaudal portion of the facet joint), and the inferior half of the jointis formed by the vertebral level above the joint (which will accordinglybe called the cephalad portion of the facet joint). For example, in theL4-L5 facet joint, the caudal portion of the facet joint is formed bysuperior articular surfaces and supporting boney structures on the L-5vertebra, and the cephalad portion of the facet joint is formed byinferior articular surfaces and supporting boney structures on the L-4vertebra.

II. Caudal Universal Facet Prosthesis

A. Structure

FIG. 3 shows a caudal facet joint replacement prosthesis 26 thatembodies features of the invention. The prosthesis 26 is designated“caudal” because it creates an artificial facet joint structure 28 forthe superior half of a facet joint replacement. The caudal prosthesis 26allows for the removal and replacement of injured, diseased and/ordeteriorating natural superior articular surfaces and supporting boneystructure on the vertebral body below the facet joint, to provideimproved support for the spinal column.

The artificial facet joint structure 28 articulates with the inferiorhalf of the facet joint, which itself can comprise the natural cephaladportions of the facet joint (i.e., inferior articular surfaces andsupporting boney structure on the vertebral body above the facet joint),or an artificial facet joint structure formed by a cephalad jointreplacement prosthesis 48 (shown in phantom lines in FIG. 3 and as willbe described. later).

As will become apparent, the prosthesis 26 is particularly well suitedto double-sided procedures and/or for procedures involving vertebralbodies that are not symmetrical.

As shown in FIG. 3, the prosthesis 26 comprises a bar element 30. Thebar element 30 is secured to a vertebral body 10 by at least onefixation element 32. As also shown in FIG. 3, the bar element 30 carriesat least one facet joint structure element 34. In FIG. 3, two fixationelements 32 (left and right) and two facet joint structure elements 34(left and right) are shown. The prosthesis 26 thereby readilyaccommodates a double-sided (i.e., left and right) caudal facet jointreplacement.

The bar element 30 is sized and shaped to span the distance between leftand right pedicles 14 of a vertebral body 10. The bar element 30 may beformed of a material commonly used in the prosthetic arts including, butnot limited to, polyethylene, rubber, titanium, chrome cobalt, surgicalsteel, bony in-growth sintering, sintered glass, artificial bone,ceramics, or a combination thereof.

As shown in FIG. 3, the spanning length of the bar element 30 may befixed. Alternatively, as shown in FIG. 4, the spanning length of the barelement 30 may be adjustable by the inclusion of an intermediate slidingjoint 36. The adjustment of the spanning length of the bar element 30may also be made, e.g., by use of a turning gear wheel mechanism, or aratchet mechanism, or the like. A surgeon thereby has the capability tosize the bar element 30, to custom-fit the prosthesis 26 during surgery.

In FIGS. 3 and 4, each end of the bar element includes an opening 38.Each opening 38 accommodates passage of the fixation element 32. In FIG.1, the fixation elements 32 take the form of pedicle screws or nails.The fixation elements 32 secure the left and right ends of the barelement 30, respectively, to the left and right pedicles 14 of thevertebral body 10.

One or both of the openings 38 could be elongated, either along thesuperior-inferior axis of the vertebral body, or transverse this axis,to allow for varying orientations and/or sizes of the fixation elements32. Passage of the fixation elements 32 through openings 38 providesstraightforward and flexible fixation of the bar element 30 to thevertebral body 10.

The bar element 30 shown in FIGS. 3 and 4 carries left and right facetjoint structure elements 34. The presence of the bar element 30 spanningbetween the facet joint structure elements 34 lends overall stability tothe prosthesis 26. Weight borne by the left and right facet jointstructure elements 34 is evenly distributed by the bar element 30between the left and right sides of the vertebral body 10.

The facet joint structure elements 34 may be fixedly attached to the barelement (e.g., by pre-welding, or by gluing with a biocompatibleadhesive) to provide a fixed, pre-ordained spaced apart relationshipbetween the facet surface elements 34. The preordained, fixed positionof and the spacing between the facet joint structure elements 34 canvary, e.g., among prostheses 26 intended for children and adults, aswell as for male and females. The physician can also select the desiredposition of the facet joint structure elements 34 based upon prioranalysis of the morphology of the targeted joint using, for example,plain film x-ray, fluoroscopic x-ray, or MRI or CT scanning.

The facet joint structure elements 28 can be located at or near thetypical anatomic position of the caudal portion of the facet joint,which would be generally aligned with the fixation element 32 (as FIG. 7shows in connection with another embodiment, which will be describedlater). Alternatively, if desired, and as shown in FIG. 3, the facetjoint structure elements 34 can be fixed within a range of pre-ordainedlocations medial of the typical anatomic location. Still alternatively,and as shown in FIG. 4, one or both of the facet joint structureelements 34 can be fixed within a range of preordained positions lateralof the typical anatomic position.

Alternatively, one or both of the facet joint structure elements 34 canbe carried by the bar element 30 for relative movement, e.g., in a track40 extending in the bar element (see FIG. 5). In this arrangement, theposition of the movable facet joint structure element(s) 34 can beinfinitely varied during surgery to match the particular anatomy of thefacet joint or joints being replaced. In this arrangement, a set screw42 or the like can secure the position of the movable facet jointstructure element 34 along the track 40.

Whether one or more of the facet joint structure elements 34 are fixedor movable relative to the bar element 30, the prosthesis 26 canaccommodate a variety of different symmetric or asymmetric anatomicsituations, e.g., where the caudal portion of the facet joint on oneside of a vertebral body 10 are differently spaced from those on theother side of the same body 10. The prosthesis 26 can include adjustablefacet joint structure elements 34, or, alternatively, it can bepre-formed with various fixed relative positions of left and right facetjoint structure elements 34, either symmetric and asymmetric; e.g., leftand right typical; or left and right medial; or left and right lateral;or left medial and right lateral, or combinations thereof.

The invention thereby makes it possible for a surgeon to install a“custom” implant during the surgical procedure.

The facet joint structure elements 34 may be made of a biocompatiblepolymer (e.g., polyethylene or rubber), or a biocompatible ceramic, orbony in-growth surface, or sintered glass, or artificial bone, or acombination thereof. The material is selected to provide glide andcushioning ability for potential contacting components. Desirably, thesesurface materials are, for strength, backed by a biocompatible metal(e.g., titanium, titanium alloys, chrome cobalt, or surgical steel).

The geometry of the caudal facet joint structure elements 34 can alsovary, depending on the geometry of the matching cephalad portions of afacet joint or facet joint replacement. As shown in FIGS. 3 and 4, thecaudal facet joint structure elements 34 may be generally concave orcup-shaped, to thereby articulate with generally convex or ball-shapedfacet joint structures of the cephalad portion of the facet joint orfacet joint replacement. Alternatively, as shown in FIG. 5, the caudalfacet joint structure elements 34 may be generally convex orball-shaped, to thereby articulate with generally concave or cup-shapedfacet joint structure of the cephalad portion of the facet jointreplacement

The ball-shaped elements and the cup-shaped elements can have variousdiameters and arcs of curvature. Preferably, the ball-shaped element iselliptical or round.

The cup-shaped elements may be any appropriate shape including, but notlimited to, rectangular, disc shaped, trough shaped, or cup shaped. Thecup-shaped element is desirably sized larger than the articulatingball-shaped half of the facet joint, to allow for motion of the joint.

Regardless of geometry, the facet joint structure elements 34 can befixedly attached or removably attached, e.g., by Morse tapers 44 (seeFIG. 5), or screws, slots, and the like.

In an alternative arrangement (see FIG. 7), the bar element 30 and facetjoint structure elements 34 form a preformed unit, and the fixationelements 34 comprise stems 46 secured to the vertebral body 10. Thestems 46 are secured by insertion into the pedicles 14 (e.g., byscrewing or tapping). In FIG. 7, there are two stems 46 inserted,respectively, into the left and right pedicles 14.

In this arrangement, the bar element 30 and facet joint structureelements 34 are desirably secured as a preformed unit with compressiveforces or friction to the stems 46, e.g., by Morse tapers 44, to therebyenable their removal and replacement as a unit. Alternatively, the barelement 30 and facet joint structure elements 34 can be fixedly attachedas a unit to the stem 46, e.g., by adhesive, screw, nail, or any meansknown in the art.

As FIGS. 7 and 8 show, the geometry of the caudal facet joint structureelements 34 on the bar element 30 attached to the stems 46 can vary, aspreviously described, depending on the geometry of the matching cephaladportions of a facet joint or facet joint replacement. As shown in FIG.7, the caudal facet joint structure elements 34 may be generally concaveor cup-shaped, to thereby articulate with generally convex orball-shaped facet joint structure of the cephalad portions of the facetjoint or facet joint replacement. Alternatively, as shown in FIG. 8, thecaudal facet joint structure elements may be generally convex orball-shaped, to thereby articulate with generally concave or cup-shapedfacet surfaces of the cephalad portions of the facet joint replacement.

The caudal prosthesis 26 for the superior half of the facet jointreplacement described above may be used as a replacement for the caudalportions of one or more of facet joints at any facet joint at any levelof the spine. In the preferred embodiment, the prosthesis 26 is used toreplace the caudal portion of one or more facet joints. The prosthesis26 is designed such that it has the appropriate cephalad and caudaddirections as well as the appropriate medial/lateral angulation for thegiven level of the spine where the implant occurs.

In further embodiments, one or more surfaces of a prosthesis 26 may becovered with various coatings such as antimicrobial, antithrombotic, andosteoinductive agents, or a combination thereof. See, e.g., U.S. Pat.No. 5,866,113, which is incorporated herein by reference. These agentsmay further be carried in a biodegradable carrier material with whichthe pores of the stem and/or cup member of certain embodiments may beimpregnated. See, e.g., U.S. Pat. No. 5,947,893, which is alsoincorporated herein by reference

In still further embodiments of the present invention, the prosthesis 26may be attached to strengthened or fortified bone. Vertebrae may bestrengthened prior to or during fixation of the prostheses using themethods, e.g., described in U.S. Pat. No. 5,827,289, which isincorporated herein by reference. This type of bone strengthening isparticularly suggested for osteoporotic patients who wish to have facetreplacement.

B. Surgical Method for Facet Replacement Using the Caudal Prosthesis

A surgical procedure removes and replaces the caudal portion of a facetjoint with the caudal prosthesis 26 described above.

In one embodiment, the surgical procedure comprises exposing the spinousprocess, lamina, and facet joints at a desired level of the spine usingany method common to those of skill in the medical arts. Prominent boneof the caudal portion of the natural facet joint natural, e.g., thesuperior articular process 22 and its supporting bone, may rongeuredusing any means common in the field. The caudal portion of the naturalfacet joint may also be trimmed to decompress the adjacent nerve root. Areamer or any other instrument that is useful for grinding or scrapingbone, may be used to ream the caudal portion of the facet joint into thepedicle, to reach the geometry shown in FIG. 3, which is suitable forreceiving the caudal prosthesis 26.

The prosthesis can then directly screwed or tapped into the vertebralbody 10 using pedicle screws or other fixation elements. Because thecaudal portion of the natural facet joint has been removed, theartificial facet joint structure of the caudal prosthesis 26 can beinstalled in a desired position and orientation, free of anatomicconstraints imposed by the preexisting articular configuration of thecaudal portion of the natural facet joint. Furthermore, the artificialfacet joint structure of the caudal prosthesis 26 can comprise anartificial articular configuration that is unlike the preexistingarticular configuration of the natural facet surface (which is removed),so that a desired articulation or bony anatomy can be totally restored.Furthermore, decompression of the adjacent nerve root can be maintained,eliminating pain at the same time a desired anatomy is restored

Further details of surgical procedures suitable for installing theprosthesis 26 are described in co-pending U.S. patent application Ser.No. 09/693,272, filed Oct. 20, 2000, and entitled “Facet ArthroplastyDevices and Methods,” which is incorporated herein by reference

III. Cephaled Prosthesis

A. Structure

FIG. 9 shows a cephalad facet joint replacement prosthesis 48 thatembodies features of the invention. The prosthesis 48 is designated“cephalad” because it provides an artificial facet joint structureelement 50 for the inferior half of a facet joint replacement. Thecephalad prosthesis 48 allows for the removal and replacement ofinjured, diseased and/or deteriorating natural inferior articularsurfaces and supporting boney structure on the vertebral body above thefacet joint, to provide improved support for the spinal column.

The artificial facet joint structure 50 articulates with the superiorhalf of the facet joint, which itself can comprise the natural caudalportions of the facet joint (i.e., the natural superior articularsurfaces and supporting boney structure on the vertebral body below thefacet joint), or an artificial facet joint structure formed by a caudaljoint replacement prosthesis 28.

In FIG. 9, the superior half of the facet joint replacement comprisesthe caudal facet joint structure 28 of the prosthesis 26 of the generaltype shown in FIGS. 3 and 4. In FIG. 9, the caudal facet joint structure28 comprise cup-shaped caudal facet joint structure elements 34, which,in FIG. 9, are mutually located on the bar element 30 in positionslateral of the typical anatomic location. As shown in FIGS. 3 and 4, therelative position of the cup-shaped caudal facet joint structureelements 34 can vary among the typical anatomic position, a positionmedial of this position, and a position lateral of this position, as hasalready been explained. The cephalad facet joint structure elements 50are positioned to align with the caudal facet joint structure elements34.

The cephalad prosthesis 48 shown in FIG. 9 desirably spans the lamina 16from the left side of a vertebral body 10 to the right side of thevertebral body 10. The cephalad prosthesis 48 thereby makes possible theremoval and replacement of injured, diseased and/or deterioratingcomponents along the cephalad portions of both left and right facetjoints, to provide improved support for the spinal column. The cephaladprosthesis 48 allows for replacement of diseased and deterioratinginferior portions of the vertebra and partial replacement of lamina,which may be pressing on the spinal nerves, to relieve pain. Thecephalad prosthesis 48 creates artificial facet joint structure elements50 for the inferior half of facet joints in the spine, which provideimproved support for the spinal column.

Because the cephalad portion of the natural facet joint is removed, theartificial facet joint structure element 50 of the cephalad prosthesis48 can be installed in a desired position and orientation, free ofanatomic constraints imposed by the preexisting articular configurationof the cephalad portion of the natural facet joint. Furthermore, theartificial facet joint structure element 50 of the cephalad prosthesis48 can comprise an artificial articular configuration that is unlike thepreexisting articular configuration of the natural facet surface (whichis removed), so that a desired articulation or bony anatomy can betotally restored. Furthermore, decompression of the adjacent nerve rootcan be maintained, eliminating pain at the same time a desired anatomyis restored.

As FIG. 9 shows, the caudal prosthesis 26, e.g., as described above, mayalso be installed with the cephalad prosthesis 48, to replace both thecaudal and cephalad portions of the natural facet joint, after bothcaudal and cephalad portions of the natural facet joint are surgicallyremoved. Together, the caudal and cephalad prostheses 26 and 48 form atotal facet replacement system 52.

Because the system 52 entails removal of both the caudal and cephaladportions of the natural facet joints, the placement of the total facetreplacement system 52 is not necessarily constrained by normal anatomicconsiderations. To attain desired alignment and load conditions, theartificial facet joint structures 28 and 48 can be positioned along,medial of, or lateral of the normal anatomic locations of the facetjoints. Likewise, the facet joint structures 28 and 48 can be positionedat or slightly superior or slightly inferior to the normal anatomiclocation of the facet joints.

The system 52 can provide a succession of entirely artificial facetjoints along a length of the spinal column. Alternatively, just theinferior half one or more facet joints, or just the superior half of oneor more facet joints, may be replaced. The inferior and/or superiorhalves of facet joints may be replaced on one side of a given vertebra(unilateral), on the both sides of a given vertebra (bilateral), or acombination of each along a length of the spinal column. At the sametime a desired bone anatomy is restored, decompression of the adjacentnerve root can be maintained to eliminate pain.

In the embodiment shown in FIG. 9, the cephalad prosthesis 48 comprisesthree separate components—a central component 54 and arm components 56and 58 (left and right).

The central component 54 may be variously constructed. In theillustrated embodiment, the central component 54 comprises a spinousprocess chimney 60 and left and right lamina plates 62 and 64. Thespinous process chimney 60 functions to surround and stabilize theposterior aspect of the spinous process 18. If desired, the spinousprocess 18 may be fixed between interior walls of the spinous processchimney 60 with a trans-spinous process screw 66.

The left and right lamina plates 62 and 64 contact the lamina 16, to aidfixation of the central component 54 to the vertebral body 10. Theplates 62 and 64 can be secured to the lamina 16 by various ways, e.g.,poly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails,bolts, anchors, break-away screws to facilitate any future removal ofthe prosthesis, or a combination thereof, or any other means known inthe art. Lamina hooks 68 are also desirably used to push against thelamina 16, thereby adding additional stability.

The left and right arm components 56 and 58 attach by a coupling tomedial sides of the left and right lamina plates 62 and 64. The couplingjunction 70 between the arm components 56 and 58 and the lamina plates62 and 64 may take a number of different forms, including, e.g., aslotted joint between the plate 62/64 and the respective arm 56/58.Alternatively, a screw attachment, a hook attachment, or a snap-fitattachment can be used

The left and right arm components 56 and 58 each include a superioropening 72. Each opening 72 accommodates passage of a fixation element74. In FIG. 9, the fixation elements 74 take the form of bilateralpedicle screws or nails, but other forms of fixation can be used.

The fixation elements 74 secure the superior portions of the left andright arm components 56 and 58, respectively, to the left and rightpedicles 14 of the vertebral body 10. One or both openings 72 could beelongated, either along the superior-inferior axis of the vertebral body10 or transverse this axis, to allow for varying orientations and/orsizes of the pedicle screw

Passage of the pedicle screws through the openings 72 providesstraightforward and flexible fixation of the left and right armcomponents 56 and 58 to the vertebral body. Lamina hooks 68 are alsodesirably used to push against the lamina 16, thereby adding additionalstability.

The left and right arm components 56 and 58 also each includes adepending cephalad facet joint structure element 76. The facet jointstructure elements 76 are sized and located for articulation with anatural caudal portion of the facet joint or an artificial caudal facetjoint structure element.

The relative position and geometry of the cephalad facet joint structureelements 76 can of course vary, depending on the relative positions andgeometry of the matching natural caudal portion of the facet joint or anartificial caudal facet joint structure element. As shown in FIG. 9, thecephalad facet joint structure elements 76 may be generally convex orball-shaped, to thereby articulate with generally concave or cup-shapedfacet joint structure elements 34 of the caudal prosthesis. Thisarticulation is also shown in phantom lines in FIGS. 3 and 4, with thefacet joint structure elements 34 and 76 arranged at alternative,different anatomic positions.

Alternatively, as shown in phantom lines in FIG. 5, the cephalad facetjoint structure elements 76 may be generally concave or cup-shaped, tothereby articulate with generally convex or ball-shaped facet jointstructure elements 34 of the caudal prosthesis

As before explained, the ball-shaped joint structure elements and thecup-shaped joint structure elements can have various diameters and arcsof curvature.

Regardless of geometry, as with the caudal facet joint structureelements 34, the cephalad facet joint structure elements 76 can befixedly attached to the respective arm component 56 and 58.Alternatively, the cephalad facet joint structure elements 76 can beremovably attached to the respective arm component 56 and 58, e.g., byMorse tapers 44, or screws, slots, and the like.

Regardless of geometry, the cephalad facet joint structure elements 76may be made of a biocompatible polymer (e.g., polyethylene or rubber),or a biocompatible ceramic, or bony in-growth surface, or sinteredglass, or artificial bone, or a combinations thereof. Desirably, thesesurface materials are backed by a biocompatible metal (e.g., titanium,titanium alloys, chrome cobalt, or surgical steel).

The central component 54 and the left and right arm components 62 may bemade of material commonly used in the prosthetic arts including, but notlimited to, polyethylene, rubber, titanium, chrome cobalt, surgicalsteel, bony in-growth sintering, sintered glass, artificial bone,ceramics, or a combination thereof.

In further embodiments, one or more surfaces of the cephalad prosthesis48 may be covered with various coatings such as antimicrobial,antithrombotic, and osteoinductive agents, or a combination thereof.See, e.g., U.S. Pat. No. 5,866,113, which is incorporated herein byreference. These agents may further be carried in a biodegradablecarrier material with which the pores of the base member and/or anyscrews, bolts, or nails of certain embodiments may be impregnated. See,e.g., U.S. Pat. No. 5,947,893, which is incorporated herein byreference.

In other arrangements, the cephalad prosthesis 48 may be attached tostrengthened or fortified bone. Vertebrae may be strengthened prior toor during fixation of the prosthesis using the methods described, e.g.,in U.S. Pat. No. 5,827,289, which is incorporated herein by reference.This type of bone strengthening is particularly suggested forosteoporotic patients who wish to have facet replacement.

B. Surgical Method for Lamina/Facet Replacement

Using the Cephalad Prosthesis

A surgical procedure removes and replaces the inferior lamina 16 and thecephalad portion of the facet joint (e.g., the articulated inferiorprocesses 24 and its supporting bone of the targeted vertebral body 10)with the cephalad prosthesis 48 as described above.

In one embodiment, the surgical procedure exposes the spinous process,lamina, and facet joints at a desired level of the spine using anymethod common to those of skill in the medical arts. The cephaladportion of the facet joint is cut at or near a selected resection line.Most of the lamina 16 is desirably preserved, as is the facet jointcapsule, which may be opened and folded back. The facet joint capsulemay be cut perpendicular to its direction. The cephalad portion of thefacet joint may then be retracted from the caudal portion. Once thefacet joint 620 is separated, the cut inferior bone of the upper joint(e.g., the cut inferior portion of the L4 vertebra in the L4-L5 joint)may be removed. Alternatively, it may be possible to remove the cutinferior bone while simultaneously separating the facet joint.

The caudal prosthesis 26 can also be installed as previously described,either before of after the inferior bone is removed or even cut.

The cephalad prosthesis 48 as described above is placed over the spinousprocess 18 over the lamina 16. The cephalad prosthesis 48 is attached asabove described to the lamina 16 and to each pedicle. The cephaladprosthesis 48 may also be further attached to the spinous process 18with a trans-spinous-process screw 66 to provide additional stability,as also previously described.

Further details of surgical procedures suitable for installing theprosthesis 48 are described in co-pending U.S. patent application Ser.No. 09/693,272, filed Oct. 20, 2000, and entitled “Facet ArthroplastyDevices and Methods,” which is incorporated herein by reference.

The size and shape of any prosthesis disclosed herein are desirablyselected by the physician, taking into account the morphology andgeometry of the site to be treated. The shape of the joint, the bonesand soft tissues involved, and the local structures that could be harmedif move inappropriately, are generally understood by medicalprofessionals using textbooks of human anatomy along with theirknowledge of the site and its disease and/or injury. The physician isalso desirably able to select the desired shape and size of theprosthesis and its placement in and/or around the joint. based uponprior analysis of the morphology of the targeted joint using, forexample, plain film x-ray, fluoroscopic x-ray, or MRI or CT scanning.The shape, size and placement are desirably selected to optimize thestrength and ultimate bonding of the prosthesis to the surrounding boneand/or tissue of the joint.

Other embodiments and uses of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. All documents referenced herein arespecifically and entirely incorporated by reference. The specificationand examples should be considered exemplary only with the true scope andspirit of the invention indicated by the following claims. As will beeasily understood by those of ordinary skill in the art, variations andmodifications of each of the disclosed embodiments can be easily madewithin the scope of this invention as defined by the following claims.

What is claimed is:
 1. A prosthetic system comprising: a bar membercomprising a left side region and a right side region, wherein the barmember comprises a track; a first fixation area on the left side region;a second fixation area on the right side region; a first fixationelement received in the first fixation area; a second fixation elementreceived in the second fixation area; a first facet joint structureelement positioned nearer to the first fixation area than the secondfixation area; and a second facet joint structure element positionednearer to the second fixation area than the first fixation area, whereinthe first facet joint structure and the second facet joint structure areeach received within the track.
 2. The system of claim 1, wherein thefirst facet joint structure element is articulatable.
 3. The system ofclaim 1, wherein the bar element comprises a fixed length.
 4. The systemof claim 1, wherein the bar element comprises a sliding joint.
 5. Thesystem of claim 1, wherein the first facet joint structure is fixed tothe bar member via welding or an adhesive.
 6. The system of claim 1,wherein the track is positioned along a midline of the bar member. 7.The system of claim 1, wherein the distance of separation between thefirst facet joint structure and the second facet joint structure canvary.
 8. The system of claim 1, wherein a first set screw secures thefirst facet joint structure along the bar member and a second set screwsecures the second facet joint structure along the bar member.
 9. Thesystem of claim 1, wherein the first facet joint structure comprises aconcave surface and the second facet joint structure comprises a concavesurface.
 10. The system of claim 9, wherein the first concave surfaceand the second concave surface face each other.
 11. The system of claim1, wherein the first facet joint structure comprises a convex surfaceand the second facet joint structure comprises a convex surface.
 12. Aprosthetic system comprising: a bar member comprising a left side regionand a right side region, wherein the bar member comprises a track; afirst fixation element received through the left side region; a secondfixation element received through the right side region; a first facetjoint structure element positioned nearer to a first fixation area thana second fixation area; and a second facet joint structure elementpositioned nearer to the second fixation area than the first fixationarea, wherein the first facet joint structure element is positionedwithin the track and the second facet joint structure element ispositioned within the track, wherein the first facet joint structureelement is moveable along the track relative to the second facet jointstructure, wherein a distance of separation between the first facetjoint structure and the second facet joint structure is adjustable. 13.The system of claim 12, wherein the bar member has a fixed length. 14.The system of claim 12, wherein the bar member has an adjustable length.15. The system of claim 14, wherein the bar member has a sliding joint.16. The system of claim 12, wherein first facet joint structurecomprises a first convex surface and the second facet joint structureelement comprises a second convex surface that faces the first convexsurface.